Ultra-thin polarization mode converters based on liquid crystal materials

ABSTRACT

A method and apparatus that includes a first waveguide segment that differentially changes the amplitude of the light relative to a first polarization orientation, a thickness of oriented liquid crystal or other birefringent material sufficient to delay one polarization component one-half wavelength relative to another, and a second waveguide segment that also differentially changes the amplitude of the light based on the polarization orientation. Also, an apparatus that includes a thin polarization converter that includes a thin first substrate that is substantially transparent to a wavelength of light, and a birefringent material deposited on one or more surfaces of the first substrate and oriented such that the polarization converter forms a half-wavelength birefringent plate for the light. Also, an apparatus having a first substrate surface, a second substrate surface, and a liquid crystal material between the first and second substrate surfaces to form a polarization converter.

RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.10/265,873 filed on Oct. 7, 2002, now U.S. Pat. No. 6,928,200 which isincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to the field of optics, and more specifically tohalf-wave polarization converters formed by e.g., sandwiching liquidcrystal material between ultra-thin substrates or within a thin slot, orby depositing birefringent material on one or both surfaces of a thinsubstrate.

BACKGROUND OF THE INVENTION

Optical amplifiers can be implemented by providing a gain medium in theoptical path of a light signal, for example, by forming an opticalwaveguide on a substrate. A gain medium typically requires an invertedpopulation, in which more atoms or molecules are in an excited statethan are in a state having less energy. A semiconductor junction canprovide the electrical pump energy needed to obtain an invertedpopulation in a gain medium. Optical pumping can also be used.

The term “laser” refers to the amplification of light by the stimulatedemission of radiation. Lasers include a gain medium and a feedbackmechanism. Energy is added to the gain medium to induce a populationinversion, wherein the lasing species has more electrons in an outershell (or high-energy state) than in an inner shell (or low-energystate). Interaction from a passing photon with a species having aninverted state can induce stimulated emission, wherein two photons areoutput, each having the same wavelength, phase, and polarization as theinput photon. This gain in the number of photons provides theamplification needed by the laser.

In a laser, an active material, for example, a semiconductor, or a glasssuitably doped with an active atomic species such as neodymium, isplaced in a cavity resonator with a feedback path formed by, e.g., tworeflecting or at least partially reflecting mirrors.

The laser will predominately output light having the characteristics(modes) that are amplified the most. These modes result from aninteraction of the optical feedback and filtering with the invertedpopulation (the characteristics of the source of energy). The modesinclude such characteristics as the spatial shape and spreading of thelaser beam, its polarization(s), wavelength(s), etc. Since there will bea fixed amount of input energy available in the laser, once one mode isamplified even slightly more than another mode, that one mode will takemore of the available energy (becoming the dominant mode), leaving lessenergy for the other modes. Various parts of the laser beam (e.g.,across its cross section) can lase in different modes. Modes can alsovary over time.

In a gain medium of a laser or an amplifier, the interaction of light inthe waveguide and the surface of the substrate can differentially filterorthogonally polarized components of the laser beam. That is, one linearpolarization mode can be suppressed, while the orthogonal polarizationmode may not be. Thus, solid-state lasers having waveguides formed atthe surface of a substrate will tend to lase with polarized modes, atleast to some extent. Amplifiers having waveguides formed at the surfaceof a substrate will tend to amplify one polarized mode more thananother, at least to some extent.

There is thus a need to provide a compensation mechanism thatcounteracts or interacts with the effect of waveguide polarization, inorder to obtain a desired overall polarization function (such as nodifferential gain between polarization modes).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a half-wave polarized plate 100 of one embodiment of thepresent invention.

FIG. 2 shows a method 200 used in the present invention.

FIG. 3A, 3B. 3C, 3D show four liquid crystal materials 310, 320, 330,340, respectively, that are used in various embodiments of theinvention.

FIG. 4A shows a representation of a liquid crystal film 400 having arandom orientation of molecules.

FIG. 4B shows a representation of a liquid crystal film 410 havinguniaxial orientation of molecules.

FIG. 5 shows an isometric view of a laser system 500 using the presentinvention.

FIG. 6 shows a side schematic view of a waveguide device 600 accordingto one aspect of the invention.

FIG. 7 shows a half-wave polarized plate 700 with high birefringentproperties deposited on at least one face of substrate 720, andpolarization-oriented in a desired direction 707.

FIG. 8A shows a polarization compensated waveguide system 800 with asingle polarization converter 820 bisecting waveguide 840.

FIG. 8B shows a polarization compensated waveguide system 801 with twopolarization converters 820 each bisecting half of waveguide 840.

FIG. 8C shows a polarization compensated waveguide system 802 having twopolarization converters 820 dividing unequal-length sections ofwaveguide 840.

FIG. 8D shows a polarization compensated waveguide system 803 with threepolarization converters 820 dividing waveguide 840.

FIG. 8E shows a polarization compensated waveguide system 804 withpolarization converter 820 dividing arrayed waveguide device 850.

FIG. 9A shows a polarization compensated waveguide system 900 with asingle polarization converter 920 bisecting waveguide device 940.

FIG. 9B shows a polarization compensated waveguide system 901 with asingle polarization converter 920 each bisecting half of waveguidedevice 941.

FIG. 9C shows a polarization compensated waveguide system 902 havingthree polarization converters 920 each bisecting a respectiveunequal-length portion of device 942.

FIG. 9D shows an enlarged detail of a portion of the device of FIG. 9C

DESCRIPTION OF PREFERRED EMBODIMENTS

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings that form a part hereof,and in which are shown by way of illustration specific embodiments inwhich the invention may be practiced. It is understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

The leading digit(s) of reference numbers appearing in the Figuresgenerally corresponds to the Figure number in which that component isfirst introduced, such that the same reference number is used throughoutto refer to an identical component which appears in multiple Figures.The same reference number or label may refer to signals and connections,and the actual meaning will be clear from its use in the context of thedescription.

Introducing a specially designed wave plate within the feedback path ofa laser can alter the phase relationship of various polarizationcomponents of the laser beam, and thus affect the mode of the laser.Such a wave plate can be used to shift the polarization relationship ina light beam.

Some wave plates work by delaying one of two orthogonally polarizedcomponents of light incident upon them. The materials in the wave plateare asymmetric relative to polarized light in that they have a differentindex of refraction in one polarization direction than the other. Theindex of refraction corresponds to the speed of light through amaterial, so the component of light having a polarization along one axisof the plate will travel faster that the component of light having theorthogonal polarization, causing a phase shift of one component relativeto the other. The “fast” axis is often indicated on the wave plate. In a“half-wave retardation plate” (as the term is used herein), the twoorthogonally polarized components of light enter the wave plate with aphase difference of zero and emerge with a phase difference of pi (180degrees), corresponding to a ½-wavelength delay. This switches thepolarization components of the beam.

In some embodiments, the present invention provides a waveguide dividedinto two substantially identical halves (such as shown in FIG. 5 below),each of which has polarization-dependent gain of about the same amount.Suppose the light beam has two initial components called an X-directionpolarization component and a Y-direction polarization component. Thegain added to the X component in the first waveguide half is differentthan the gain added to the Y component. The half-wave polarized plate isplaced in the light path between the two waveguide halves, and switchesthe polarization components of the beam. Since the X-directionpolarization component has an amount of X-directionpolarization-sensitive gain added to it by the first half waveguide, andis then switched to the Y-direction by the half-wave polarized plate andthen has an amount of Y-direction polarization-sensitive gain added toit by the second half waveguide, and the Y-direction polarizationcomponent has an amount of Y-direction polarization-sensitive gain addedto it by the first half waveguide, and is then switched to theX-direction by the half-wave polarized plate and then has an amount ofX-direction polarization-sensitive gain added to it by the second halfwaveguide, both components are provided the same amount of gain. Theoutput beam will have gain that is the average of the X-direction gainand the Y-direction gain of the two halves of the waveguide. Thus, theoverall waveguide-plate device has gain that ispolarization-independent.

In some embodiments, passive devices have waveguides that incur“polarization-sensitive loss” that is compensated for by using thismethod as well. Such passive photonic devices include arrayed waveguidegratings and other integrated photonic devices havingpolarization-sensitive loss.

In some embodiments, the removal of polarization sensitivity ofwaveguide gain or loss is critical to the functionality of certainplanar photonic devices.

In some embodiments, the half-wave polarized plate has a polarizationorientation that is pointed at a forty-five degree angle relative to theplane of the surface of the waveguide substrate. Such an orientationprovides the desired switching of the X and Y polarization components.

In some embodiments, a simple, straight waveguide is provided for thefirst half waveguide, and a substantially identical simple, straightwaveguide is provided for the second half waveguide. In otherembodiments, a more complex waveguide structure, such as an arrayedwaveguide grating, is used for each of the two substantially identicalhalves, thus eliminating the polarization dependence of the overallstructure. Equivalently, a complex waveguide structure is divided intotwo substantially identical halves, perhaps having multiple waveguidesin each half meeting in the middle, and a half-wave retardation plate isplaced in that middle location across all the waveguides there.

FIG. 1 shows a half-wave polarized plate 100 of one embodiment of thepresent invention having a liquid crystal material 110 with highbirefringent properties spread between two thin substrate plates 120 and122 and oriented in a desired direction. In some embodiments, theorientation direction is pointed at a forty-five degree angle relativeto the plane of the surface of the waveguide into which half-wavepolarized plate is placed.

In some embodiments, thin substrate plates 120 and 122 are each made ofglass approximately twenty-five to thirty microns (micrometers) thick.In some embodiments, standard liquid crystal materials having a typicalbirefringence of 0.15 to 0.20 are used. The thickness of the liquidcrystal layer 110 depends on the amount of birefringence of the materialused. In some embodiments, the liquid crystal layer 110 is oriented (toprovide a polarization orientation that will be forty-five degrees tothe surface plane of the waveguide).

In other embodiments, substrate 120 is a thin glass sheet, and substrate122 is a thin polymer sheet (such as polyamide), in order to form athinner half-wave polarization converter plate 100. In some embodiments,thinner polarization converters reduce insertion loss of the device. Inyet other embodiments, polymer sheets are used for both substrate plates120 and 122. In yet other embodiments, quartz sheets are used for bothsubstrate plates 120 and 122.

FIG. 2 shows a method 200 used in the present invention. In someembodiments, orientation is achieved by preparing and cleaning 210 thesurfaces of substrate plates 120 and 122, depositing 220 a layer of theliquid crystal material 110 onto the prepared surface of substrate plate120 with a target thickness of two to five microns, placing 230 the topsubstrate plate 122 onto the liquid crystal material 110 and providingshear 240 of the top plate 122 relative to the bottom plate 120 (motionof the plane in a parallel direction) in order to orient molecules ofthe liquid crystal material film 110, and sealing 250 the top plate 122to the bottom plate 120. In some embodiments, the resulting sandwich isdiced 260 to a desired size, e.g., 1 cm by 15 cm. In some embodiments, agroove is formed 270 at the center of the waveguide structure and thediced portion is placed 280 in the groove.

FIGS. 3A, 3B, 3C, and 3D show four liquid crystal materials 310, 320,330, 340, respectively, that are used in various embodiments of theinvention. In some embodiments, liquid crystal material 310,methoxybenzilidene butylanaline (“MBBA”), is used for layer 110 ofFIG. 1. In other embodiments, liquid crystal material 320,p-decyloxybenzylidene p′-amino 2-methylbutylcinnamate (“DOBAMBC”), isused for layer 110 of FIG. 1. In yet other embodiments, liquid crystalmaterial 330, CH3—O-Ph-CH═N-Ph-C4H9, is used for layer 110 of FIG. 1. Instill other embodiments, liquid crystal material 340, C5H11-Ph-Ph-CN, isused for layer 110 of FIG. 1. Other embodiments use other liquid crystalmaterials.

FIG. 4A shows a representation of a liquid crystal film 400 having arandom orientation of molecules.

FIG. 4B shows a representation of a liquid crystal film 410 havinguniaxial orientation of molecules, due to, e.g., shear stress resultingfrom a surface pretreatment of the materials used for one or both of thefacing walls into which the film is formed.

FIG. 5 shows a perspective schematic view of a waveguide device system500 according to one aspect of the invention. System 500 in enclosure501 includes a waveguide 512 formed at surface 508 of substrate 510.Total reflecting mirror 518 and partially reflecting mirror 516 andwaveguide 512 form a laser cavity 502. Polarization converter 520(implemented, for example, as half-wave polarized plate 100 of FIG. 1)is placed in slot 514 that is formed at substantially the midpoint ofwaveguide 512. The polarization direction 507 is at a forty-five degreeangle to surface 508. In this embodiment, the output light beam of laser502 is further processed by output optics such as lens 541 and lens 542,and amplitude modulator 543, which is controlled by electronics 544 to,e.g., impart a data stream onto the light. Power supply 530 providespump power (via either electrical or optical energy) to the laser 502.System 500 is one example of an entire system built around apolarization converter 520 such as device 100 of FIG. 1. In otherembodiments, polarization converter 520 is implemented in other ways,such as polarization converter 620 of FIG. 6, or polarization converter700 of FIG. 7.

FIG. 6 shows a perspective schematic cross-section view of a waveguidedevice 600 according to one aspect of the invention. Device 600 includesa waveguide 640 formed as a core between an upper cladding layer 630 anda lower cladding layer 650, which is formed on a substrate 660. Avertical groove or slot 614, having side faces 611 and 613, is formed ata midpoint of waveguide 640 (e.g., through clad 630, through waveguide640 and possibly partially into clad 650), and filled with liquidcrystal material 610 to form polarization converter 620. Note that inthe finished product, liquid crystal material 610 is typicallysolidified to make permanent its polarization direction, rather thanremaining a liquid, but will still be referred to as a “liquid” crystalmaterial. Groove 614 is made thick enough to provide the properthickness of film 610. In some embodiments, surface contact 612 is madeto film 610 and shear is applied in the direction shown (i.e., parallelto the length of the groove and perpendicular to the cross-sectionalface 608 into the FIG. 6 drawing and to the length axis of waveguide640, in order to orient the molecules at a forty-five degree angle totop face surface 609 of device 600. In other embodiments, the innersurfaces of groove 614 are stressed, crystal oriented, or polished at aforty-five degree angle such that the molecules of film 610 alignthemselves to that forty-five degree angle relative to surface 609. Inthis way, the alignment of the molecules is self-aligned into a grooveof the appropriate dimensions to achieve a birefringent half-wavepolarization converter for waveguide 640.

In some embodiments, groove 614 is less than two microns thick, in orderto provide an ultra-thin transverse electric/transverse magnetic (TE/TM)polarization converter, used, without introducing significant loss, toremove polarization sensitivities of optical circuits. In someembodiments, an arrayed waveguide grating device (AWG device) having acomplex waveguide configuration is bisected by a groove 614 in orderthat polarization sensitivities of the AWG are minimized. In otherembodiments, an optical switch is bisected.

In some embodiments, a liquid crystal material, such as one selectedfrom the four shown in FIG. 3, is dissolved in an appropriate solvent,and deposited in slot 614. The solvent is then evaporated leaving a thinbirefringent layer. The thickness and polarization orientation iscontrolled to provide half-wave relative retardation to the propagatingoptical radiation. In some embodiments, slot 614 is filled with thebirefringent liquid crystal material. In other embodiments, a thin layerof the birefringent liquid crystal material is formed on both faces ofslot 614, and an optical index-matching material is used to fill theremaining portion of slot 614.

In some embodiments, a liquid crystal material having a birefringence ofdelta n=0.5 is used, and a total thickness of the half-wave plate filmis 1.55 micrometers is used for 1550 nm wavelength infrared light(l_(hwp)=wavelength/2 times delta n). In some embodiments, the insertionloss of a thin polarization converter 612 is approximately less than0.05 dB. In contrast, one conventional polarization converter made fromstressed polyimide is approximately 14.5 microns thick, has abirefringence of about 0.053, and results in an excess loss of about 0.4dB. Another reported polarization converter medium is 92-micron thickcrystal quarts having an excess loss of as much as 5 dB.

Note that polarization converter 620 is oriented to swap thepolarization components of the light beam that are amplified orattenuated by different amounts in waveguide 640. Typically, thosepolarization components are the component parallel to surface 609 andthe component perpendicular to surface 609 (e.g., where theperpendicular component has the maximum gain and the parallel componenthas the minimum gain, or where the parallel component has the maximumgain and the perpendicular component has the minimum gain) and for thatcase, polarization converter 620 is oriented at a forty-five degreeangle to surface 609. For other cases, polarization converter 620 isoriented at a forty-five degree angle to the component with the maximumgain (or minimum attenuation).

FIG. 7 shows a half-wave polarized plate 700 of one embodiment of thepresent invention having a birefringent material 710 with highbirefringent properties deposited on at least one side of substrate 720,and polarization-oriented in a desired direction 707. In someembodiments, the orientation direction 707 is pointed at a forty-fivedegree angle relative to the polarization component 732 having theminimum gain (i.e., in FIG. 7, the propagation direction 730 of a lightbeam along a waveguide has minimum gain in direction 732 and maximumgain in direction 731 perpendicular to direction 732). For example, insome embodiments, the direction 707 is at a forty-five degree angle to asurface adjacent to waveguide 512 (see FIG. 5, but where polarizationconverter 520 is replaced by a plate 700 of FIG. 7). In otherembodiments, the polarization direction 707 is pointed at an angle thatis half way between the polarization direction of maximum gain and thepolarization direction of minimum gain.

In some embodiments, birefringent material 710 is deposited on one sideof substrate 720, and an equivalent thickness of birefringent material712 is deposited on an opposing side of substrate 720, wherein thethickness of birefringent material 710 and the thickness of birefringentmaterial 712 are specific to provide an overall half-wave polarizedplate (i.e., where a first polarization component is delayed one-halfwavelength relative to the other polarization component that isperpendicular to the first). In other embodiments, birefringent material710 is deposited on only one side of substrate 720 to a thickness toprovide a half-wave polarized plate.

In some embodiments, substrate 720 is glass. In some such embodiments, aglass substrate of about 25 microns thick is used. In other embodiments,substrate 720 is quartz, or a thin polymer film such as Kapton(R) orpolyethylene. In some such embodiments, a polymer thickness of about 15microns is used.

In some embodiments, the birefringent material is a film of TiO2 havinga birefringence of 0.247. In some such embodiments, a film thickness(i.e., the total thickness of films 710 and 712, or of film 710 if onlyone side has such a film) of about 3 microns is used to providehalf-wave retardation at 1550 nm. In other embodiments, the birefringentmaterial is a film of LiNbO3 having a birefringence of 0.073 (in somesuch embodiments, a film of about 10 microns is used). In otherembodiments, the birefringent material is a film of Ta2O5. In stillother embodiments, a liquid crystal material such as described above isused (i.e., applies in a solvent, and the solvent then evaporated toleave the solid oriented liquid crystal material film on one or bothsides of substrate 720).

FIG. 8A shows a polarization compensated waveguide system 800 with asingle polarization converter 820 bisecting waveguide 840. In variousembodiments of the devices of FIGS. 8A, 8B, 8C, and 8D, polarizationconverter 820 is the polarization converter 100 of FIG. 1, polarizationconverter 620 of FIG. 6, or polarization converter 700 of FIG. 7. FIG.8A is the general case of the devices described above, wherein thewaveguide segment 841 has the same gain or attenuation characteristicsas waveguide segment 842. In some embodiments, waveguide segment 841 isthe same length as waveguide segment 842 and has a polarizationsensitivity that is perpendicular to a substrate surface adjacent towaveguide 840, and the polarization orientation of polarizationconverter 820 is at a forty-five degree angle to that substrate surface.

FIG. 8B shows a polarization compensated waveguide system 801 with twopolarization converters 820 each bisecting respective halves ofwaveguide 840. In such a system, the left-hand polarization converter820 swaps the vertical and horizontal polarization components (relativeto a substrate surface denoted as horizontal), and the right-handpolarization converter 820 re-swaps the vertical and horizontalpolarization components back to their original orientations. Thus, anypolarization in the original beam is restored, while the gain orattenuation imparted is equal to all polarization components. In theembodiment shown, waveguide segment 843 is one quarter of the entirewaveguide length, waveguide segment 844 is one half of the entirewaveguide length, and waveguide segment 845 is one quarter of the entirewaveguide length. Since sum of the lengths of segments 843 and 845(where the light beam has its original polarization orientation) isequal to the length of segment 844 (where the light beam has itsvertical and horizontal polarization components swapped), bothpolarization components have the same gain (i.e., if waveguide 840 is again medium) or attenuation (if waveguide 840 is a passive device). Insome such embodiments, the polarization orientations 807 of polarizationconverters 820 are each at forty-five degrees relative to a surfaceadjacent to waveguide 840.

FIG. 8C shows a polarization compensated waveguide system 802 having twopolarization converters 820 dividing unequal-length sections ofwaveguide 840. This configuration is similar to that of FIG. 8B in thatthe sum of lengths of segments 846 and 848 equal the length of segment847, however segment 846 has a different length than segment 848. Theresult, though, is the same as FIG. 8B in providing the same gain orattenuation to both polarization components, while maintaining thepolarization orientation of the input beam to the output beam.

FIG. 8D shows a polarization compensated waveguide system 803 with threepolarization converters 820 dividing waveguide 840. In some suchembodiments, the polarization orientations 807 of polarizationconverters 820 are each at forty-five degrees relative to a surfaceadjacent to waveguide 840. This configuration swaps the polarizationcomponents three times, thus the output beam has the polarizationcomponents swapped relative to the input beam, as was the case for FIG.8A. In some such embodiments, the polarization orientation 807 of thethree polarization converters 820 are each at forty-five degreesrelative to a surface adjacent to waveguide 840, and each waveguidesegment is the same length. In other embodiments, other numbers ofpolarization converters 820 are used, and/or different length segmentsare formed.

FIG. 8E shows a polarization compensated waveguide system 804 withpolarization converter 820 dividing arrayed waveguide (AWG) device 850.In some such embodiments, AWG device 850 is bisected along each path,wherein each path has a slightly different length than the other paths.In other embodiments, two or more polarization converters 820 are used,in a manner similar to that of FIG. 8B.

FIG. 9A shows a polarization compensated waveguide system 900 with asingle polarization converter 920 bisecting waveguide device 940.Waveguide device is an arrayed waveguide device having a single inputwaveguide 941, a splitter 942, a plurality of unequal-length curvedwaveguide sections 943 each of which is bisected by ½ wave retardationplate (polarization converter plate) 920, combiner 944, and a pluralityof output waveguides 945 each of which will output a slightly differentwavelength of light extracted from the input light from waveguide 941.In this schematic, the wavelengths are markedred-orange-yellow-green-blue-violet, but in some embodiments, thewavelengths are much closer (on the order of 1 nanometer or lessseparating the color on each output waveguide 945), and more or fewerwaveguides 943 and 945 are provided.

FIG. 9B shows a polarization compensated waveguide system 901 with asingle polarization converter 920 each bisecting half of waveguidedevice 941. This embodiment provides a plurality of input waveguides 941each carrying one or more wavelengths. The input wavelengths can be inany order, but the device will separate the wavelengths and the outputwavelengths will be ordered from longest to shortest wavelength on therespective output waveguides 945. As described for FIG. 9A, apolarization converter device 920 is formed in the middle of each of theplurality of curved waveguides 943, such that the polarizationdifferential inserted by the left-hand portion of device 940 iscompensated by the polarization differential in the right-hand portionof device 940. In other embodiments, each segment 941 and 945 arebisected by a polarization converter 920, as shown in FIG. 9C.

FIG. 9C shows a polarization compensated waveguide system 902 havingthree polarization converters 920 each bisecting a respectiveunequal-length portion of device 942. Input waveguide 941 is bisected bya polarization converter 920A, placed at a location that compensates forpolarization differential losses (or phase changes) in that portion ofthe device, each curved waveguide 943 has a slightly different lengththan the others, and each is bisected by a polarization converter 920B,and each output waveguide 945 is bisected by polarization converter920C. In some embodiments, polarization converters 920A and 920C areplaced in a position that also compensates for polarization differentiallosses of splitter 942 and combiner 944.

FIG. 9D shows an enlarged detail of a portion of the device of FIG. 9C.

CONCLUSION

One aspect of the present invention provides a method that includespassing light through a first waveguide segment that differentiallychanges the amplitude and/or phase of the light relative to a firstpolarization orientation, passing the light through a thickness oforiented liquid crystal material sufficient to delay a firstpolarization component one-half wavelength relative to a secondpolarization component that is perpendicular to the first, wherein thefirst polarization component is at a forty-five degree angle to thefirst polarization orientation, and passing the light through a secondwaveguide segment that differentially changes the amplitude and/or phaseof the light based on the polarization orientation.

In some embodiments of the method, the thickness of oriented liquidcrystal material is sandwiched between a first thin transparentsubstrate and a second thin transparent substrate, such as shown in FIG.1.

In some embodiments of the method, the thickness of oriented liquidcrystal material is deposited in a slot in a waveguide, such as shown inFIG. 6.

In some embodiments of the method, a portion of the thickness oforiented liquid crystal material is deposited on one face of a firstthin transparent substrate and the remaining portion is deposited on anopposing face, such as shown, for example in FIG. 7.

Some embodiments of the method, further include amplifying the light inthe first waveguide segment and in the second waveguide segment; andproviding feedback to form a laser, such as shown in FIG. 5.

In some embodiments of the method, the liquid crystal material swapslight having the first polarization orientation with light having apolarization orientation perpendicular to the first polarizationorientation to compensate for a polarization-sensitive differential gainof the first and second waveguide segments. In some such embodiments,the first segment is substantially equal in length to the secondsegment, wherein the waveguide is formed substantially parallel to asurface of a substrate, and wherein a polarization component of thelight parallel to the surface of the substrate is swapped with apolarization component of the light perpendicular to the surface of thesubstrate by the polarization converter, such as shown in FIG. 8A.

The present invention also provides an apparatus that includes a thinpolarization converter that includes a thin first substrate that issubstantially transparent to a wavelength of light, and a birefringentmaterial deposited on one or more surfaces of the first substrate andoriented such that the polarization converter is substantiallytransparent to, and forms a half-wavelength birefringent plate for, thewavelength of light. For example, see FIG. 7.

In some embodiments of this apparatus, a first thickness of thebirefringent material is deposited on a first major surface of the thinsubstrate and an equivalent thickness of the birefringent material isdeposited on a second major surface the substrate opposite the firstmajor surface of the substrate.

Some embodiments further include a gain medium and a feedback mechanismconfigured such that a light path through the gain medium, thepolarization converter, and the feedback mechanism forms a laser.

Other embodiments further include a waveguide having a first segment anda second segment, wherein the polarization converter is placed betweenfirst segment and the second segment to compensate for apolarization-sensitive differential gain of the waveguide. In some suchembodiments, the first segment is substantially equal in length to thesecond segment, wherein the waveguide segments are formed substantiallyparallel to a surface of a substrate, and wherein a polarizationcomponent of the light parallel to the surface of the third substrate isswapped with a polarization component of the light perpendicular to thesurface of the substrate by the polarization converter.

The present invention also provides an apparatus that includes a thinpolarization converter that includes a first substrate surface, a secondsubstrate surface, and a liquid crystal material sandwiched between thefirst substrate surface and the second substrate surface such that thepolarization converter is substantially transparent to, and forms ahalf-wavelength birefringent plate for, a wavelength of light. See, forexample, FIGS. 1 and 6.

In some embodiments, the first substrate surface is a major surface of afirst thin substrate and the second substrate surface is a major surfaceof a second thin substrate. See, for example, FIG. 1.

Some embodiments further include a gain medium, and a feedback mechanismconfigured such that a light path through the gain medium, thepolarization converter, and the feedback mechanism forms a laser.

Some embodiments of this apparatus further include a waveguide having afirst segment and a second segment, wherein the polarization converteris placed between first segment and the second segment to compensate fora polarization-sensitive differential gain of the waveguide.

In some embodiments, the first segment is substantially equal in lengthto the second segment, wherein the waveguide segments are formedsubstantially parallel to a surface of a substrate, and wherein apolarization component of the light parallel to the surface of the thirdsubstrate is swapped with a polarization component of the lightperpendicular to the surface of the substrate by the polarizationconverter.

Some embodiments of the apparatus (such as shown in FIG. 6), furtherinclude a substrate 601 having a waveguide 640 formed therein, andhaving a slot 614 transverse to the waveguide 640, wherein the firstsubstrate surface is one face 611 of the slot and the second substratesurface is an opposing face 613 of the slot, and wherein thepolarization converter 620 is formed in the slot 614 in the substrate601 by the liquid crystal material 610 sandwiched between the faces ofthe slot.

In some embodiments, the faces of the slot are pre-treated such that theliquid crystal material self aligns to an orientation that swaps a firstpolarization component of the light with a second polarization componentof the light that is perpendicular to the first polarization component.

In some embodiments, the waveguide includes a gain medium, and theapparatus further includes a feedback mechanism, such as shown in FIG.5, configured such that a light path through the gain medium, thepolarization converter, and the feedback mechanism forms a laser.

In some embodiments, the waveguide includes a first segment and a secondsegment, wherein the polarization converter is placed between firstsegment and the second segment to compensate for apolarization-sensitive differential gain of the waveguide.

In some embodiments, the waveguide includes a first segment and a secondsegment, wherein the polarization converter is placed between firstsegment and the second segment to compensate for apolarization-sensitive propagation loss of the waveguide.

In some embodiments, the first segment is substantially equal in lengthto the second segment, wherein the waveguide is formed substantiallyparallel to a surface of the substrate, and wherein a polarizationcomponent of the light parallel to the surface of the substrate isswapped with a polarization component of the light perpendicular to thesurface of the third substrate by the polarization converter.

It is understood that the above description is intended to beillustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the invention should, therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

1. An apparatus comprising: a thin polarization converter that includes:a first substrate surface; a second substrate surface; a liquid crystalmaterial sandwiched between the first substrate surface and the secondsubstrate surface such that the polarization converter is substantiallytransparent to, and forms a half-wavelength birefringent plate for, awavelength of light; a gain medium; and a feedback mechanism configuredsuch that a light path through the gain medium, the polarizationconverter, and the feedback mechanism forms a laser.
 2. An apparatuscomprising: a thin polarization converter that includes: a firstsubstrate surface; a second substrate surface; a liquid crystal materialsandwiched between the first substrate surface and the second substratesurface such that the polarization converter is substantiallytransparent to, and forms a half-wavelength bireflingent plate for, awavelength of light; a waveguide having a first segment and a secondsegment, wherein the polarization converter is placed between the firstsegment and the second segment to compensate for apolarization-sensitive differential gain of the waveguide; and whereinthe first segment is substantially equal in length to the secondsegment, wherein the waveguide segments are formed substantiallyparallel to a surface of a substrate, and wherein a polarizationcomponent of the light parallel to the surface of the substrate isswapped with a polarization component of the light perpendicular to thesurface of the substrate by the polarization converter.
 3. An apparatuscomprising: a thin polarization converter that includes: a firstsubstrate surface; a second substrate surface; a liquid crystal materialsandwiched between the first substrate surface and the second substratesurface such that the polarization converter is substantiallytransparent to, and forms a half-wavelength birefringent plate for, awavelength of light; and a substrate having a waveguide formed therein,and having a slot transverse to the waveguide, wherein the firstsubstrate surface is one face of the slot and the second substratesurface is an opposing face of the slot, and wherein the polarizationconverter is formed in the slot in the substrate by the liquid crystalmaterial sandwiched between the faces of the slot.
 4. The apparatus ofclaim 3, wherein the faces of the slot are pro-treated such that theliquid crystal material self aligns to an orientation that swaps a firstpolarization component of the light with a second polarization componentof the light that is perpendicular to the first polarization component.5. The apparatus of claim 3, wherein the waveguide includes a gainmedium, the apparatus further comprising: a feedback mechanismconfigured such that a light path through the gain medium, thepolarization converter, and the feedback mechanism forms a laser.
 6. Theapparatus of claim 3, wherein the waveguide includes a first segment anda second segment, wherein the polarization converter is placed betweenthe first segment and the second segment to compensate for apolarization-sensitive differential gain of the waveguide.
 7. Theapparatus of claim 6, wherein the first segment is substantially equalin length to the second segment, wherein the waveguide is formedsubstantially parallel to a surface of the substrate, and wherein apolarization component of the light parallel to the surface of thesubstrate is swapped with a polarization component of the lightperpendicular to the surface of the substrate by the polarizationconverter.
 8. A system comprising: a total reflecting minor; a partiallyreflecting mirror; a waveguide between the total reflecting minor andthe partially reflecting minor to form a laser cavity with the totalreflecting minor and the partially reflecting minor; and a polarizationconverter located in a slot in the waveguide, the polarization converterincluding a thickness of solid liquid crystal material sufficient todelay a first polarization component one-half wavelength relative to asecond polarization component that is perpendicular to the firstpolarization component.
 9. The system of claim 8 wherein the thicknessof solid liquid crystal material is sandwiched between a first thintransparent substrate and a second thin transparent substrate.
 10. Thesystem of claim 8 wherein the waveguide is located at a surface of asubstrate, and the polarization converter has a polarization directionthat is at a forty-five degree angle to the surface of the substrate.11. The system of claim 8 wherein: the waveguide is in a substrate andthe slot is transverse to the waveguide between a first waveguidesegment and a second waveguide segment, and wherein the solid liquidcrystal material swaps light having a first polarization orientationwith light having a polarization orientation perpendicular to the firstpolarization orientation to compensate for a polarization-sensitivedifferential gain of the first and second waveguide segments; the firstwaveguide segment is substantially equal in length to the secondwaveguide segment, wherein the waveguide is formed substantiallyparallel to a surface of the substrate, and wherein a polarizationcomponent of the light parallel to the surface of the substrate is to beswapped with a polarization component of the light perpendicular to thesurface of the substrate by the polarization converter; and thepolarization converter has a polarization direction that is at aforty-five degree angle to the surface of the substrate.
 12. The systemof claim 8, further comprising: a first lens located adjacent to anoutput of the laser cavity; a second lens located between the first lensand the laser cavity; an amplitude modulator coupled to electronics tobe controlled to impart a data stream onto light generated by the lasercavity, the amplitude modulator being located between the first lens andthe second lens; and a power supply coupled to supply power to the lasercavity.
 13. A waveguide comprising: a first polarization converterincluding a liquid crystal material and located in a waveguide; a secondpolarization converter including a liquid crystal material and locatedin the waveguide; a total reflecting mirror; and a partially reflectingmirror, the waveguide being located between the total reflecting mirrorand the partially reflecting mirror to form a laser cavity.
 14. Anarrayed waveguide device comprising: a plurality of waveguide paths,each waveguide path having a slightly different length than the otherwaveguide paths; a first polarization converter including a liquidcrystal material, the first polarization converter dividing eachwaveguide path; and wherein each of the waveguide paths is a curvedwaveguide path.
 15. The arrayed waveguide device of claim 14 wherein thefirst polarization converter is located such that a polarizationdifferential inserted by an input portion of the waveguide paths iscompensated by a polarization differential of an output portion of thewaveguide paths.
 16. An arrayed waveguide device comprising: a pluralityof waveguide paths, each waveguide path having a slightly differentlength than the other waveguide paths; a first polarization convenerincluding a liquid crystal material, the first polarization converterdividing each waveguide path; a single input waveguide path; a splittercoupled to the single input waveguide path; a plurality of unequallength curved waveguide paths coupled to the splitter, each curvedwaveguide path being bisected by the first polarization converter; acombiner coupled to the curved waveguide paths; a plurality of outputwaveguide paths coupled to the combiner; a second polarization converterincluding a liquid crystal material, the second polarization converterbisecting the input waveguide path to compensate for polarizationdifferential losses; and a third polarization converter including aliquid crystal material, the third polarization converter bisecting eachoutput waveguide path.